This invention relates to high frequency heating apparatus such as microwave ovens wherein DC power obtained by rectifying, by use of a rectification circuit, a high frequency voltage delivered from an inverter circuit and stepped up by a transformer, thereby driving a magnetron, and more particularly to such a high frequency heating apparatus provided with means for detecting an abnormal condition of an electric circuit for applying voltage to the anode of the magnetron.
A conventional high frequency heating apparatus of the above-described type comprises rectification means for converting a commercial AC power voltage to a DC voltage and inverter means including a switching element and a transformer which also serves as inductance coil means. The DC voltage from the rectification means is applied to the transformer through the switching element turned on and off so that a high frequency voltage is obtained at the inverter means. The high frequency heating apparatus further comprises a magnetron connected to a high frequency rectifying circuit. The high frequency rectifying circuit rectifies the AC voltage from the secondary winding of the transformer to thereby obtain a DC voltage which is applied to the magnetron. A control circuit is provided for controlling "on" and "off" periods of the switching element so that a magnetron anode current is maintained at a predetermined value. The inverter means is provided for the purpose of reducing the size of the transformer employed to obtain the high voltage which is applied to the magnetron.
In accordance with the above-described high frequency heating apparatus, the magnetron is oscillated upon application of the DC voltage thereto. As a result, microwave energy is generated by the magnetron and radiated to foodstuffs to be heat cooked. When the magnetron is continuously driven, a permanent magnet of the magnetron is heated as the result of generation of heat by the magnetron. Consequently, an amount of magnetic flux induced is reduced with the increase in the temperature of the permanent magnet. An amount of anode current of the magnetron is increased with the temperature increase of the permanent magnet and accordingly, an amount of microwave energy generated is also increased therewith. In order to prevent the amount of the magnetron anode current from being uselessly increased, a current transformer is conventionally provided across a primary winding input line of the transformer for detecting a transformer input current and current control means is provided for controlling the switching element of the inverter means in response to a current detection signal from the current transformer so that the magnetron anode current is maintained at a predetermined value.
On the other hand, it has been desired that measures be taken against an abnormal condition such as a burnout in a transformer secondary circuit or the circuit for applying voltage to the magnetron anode. Although one of the measures is to detect the input current to a primary winding of the transformer to thereby detect the abnormal condition, it is impossible to do so for the reason that the above-mentioned current control means for controlling the magnetron anode current is operated so that the increase and decrease in the primary current is restrained.